1. Field of the Invention
The present invention relates generally to color display monitors and more particularly to the degaussing of color monitors.
2. Description of the Related Art
An ambient magnetic field can degrade the display of color monitors (cathode-ray tubes) because the magnetic field alters the screen landing point of the scanned electronic beams. In these monitors, phosphor dots of primary colors, e.g., red, green and blue, are typically deposited on the viewing screen in a pattern of dot triads with a phosphor dot of each color forming one-third of each triad. A scanned electron beam is generated for each color and when the dots are excited by their respective electron beams, they phosphoresce to produce a full-color picture. The dots are carried on a phosphor-dot faceplate which may be a portion of the picture tube itself or a separate internal glass plate.
One form of a beam-directing device is a shadowmask which is a thin perforated plate that is spaced inwardly from the phosphor-dot faceplate. Each aperture of the shadowmask is associated with a different one of the phosphor triads and is positioned to allow each dot of that triad to be excited only by its respective electron beam.
In accordance with Lorentz's Law, a deflecting force is imposed on the electrons of the electron beams as they pass through an ambient magnetic field. This deflecting force can cause the electron beams to become misaligned with their respective phosphor dots with consequent display color degradation, e.g., loss of color purity, picture rotation and picture translation.
Color display distortion is particularly troublesome in environments which have strong, changing magnetic fields, e.g., naval warships and medical magnetic resonance imaging (MRI). Although movement of naval warships through the earth's magnetic field is one source of magnetic distortion, a more powerful distortion source is the external magnetic field that is often purposely generated to oppose this natural field and thus hide the ship from enemy magnetic sensors, e.g., magnetically actuated mines.
To mitigate the distorting effects of ambient magnetic fields, color monitors typically include a metal shield that is positioned inside the monitor's cathode-ray tube. If the magnetic domains of this internal shield are initially oriented in opposition with the ambient magnetic field, that field is substantially reduced and the internal shield is effective in preventing undesirable electron beam deflection. If the ambient field then changes to a new orientation or magnitude, hysteresis effects in the internal shield will prevent complete realignment of its domains in opposition to the new orientation. Consequently, a remnant field exists that may be strong enough to cause significant misalignment of the electron beams with their respective phosphor dots.
The act of allowing the local domains of the internal shield and shadowmask to realign in opposition to the reoriented ambient magnetic field is referred to as degaussing (demagnetizing). Typical degaussing systems run alternating currents through a degaussing coil that is arranged to envelope the cathode-ray tube in its magnetic field. Monotonically reducing the initial high strength of this alternating field to a small value, prior to its termination, enhances the ability of the internal shield domains to realign in opposition to the ambient field.
In an exemplary system, a degaussing coil is arranged in series with a thermistor, an alternating voltage source (usually, the AC line voltage) and a switch. When the switch is closed, a sinusoidal current is sent through the degaussing coil. The magnetic field strength declines because the thermistor resistance increases as it is heated by the series current. The degaussing time of thermistor-based systems is typically slow, e.g., 1-3 seconds, because it is dictated by the thermistor thermal time constant. In addition, the time before these systems can be degaussed again is limited by the time it takes the thermister to cool down, e.g., 5 to 10 minutes.
Thermistor-based degaussing systems generally require considerable power, e.g., typical peak power of 1000 watts, and their high surge currents can cause electromagnetic interference (EMI) problems in associated circuits. In general, the performance parameters of thermistor-based systems are poorly defined because they are susceptible to variation in line voltage and variation of the thermistor resistance with ambient temperature.
Improved degaussing methods should be fast, e.g., &lt;500 milliseconds, quickly repeatable, e.g., &lt;10 seconds and efficient. Preferably, they should not require monitor blanking nor leave significant residual magnetism and they should minimize EMI production by avoiding high surge currents. They should be relatively insensitive to line voltage variations and temperature variations, inexpensive and compatible with a variety of monitors, e.g., commercial-off-the-shelf (COTS) monitors.